Chromatography is a valuable tool for separating, quantifying and identifying chemical compounds. In a High Performance Liquid Chromatography (HPLC) system, a liquid sample containing a target compound is introduced into a column under pressure. The target compound is introduced into a mobile phase, which is contacted with a stationary phase. The speed of travel through the column depends on the mobile phase non-covalent interactions of the target compound with the stationary phase. HPLC systems utilize various detectors capable of generating a signal that varies with the target compound concentration eluting from the separation column. An example of an HPLC detector is the CARONA CAD manufactured by ESA of Chelmsford, Mass., USA. The CARONA CAD includes a nebulizer that receives a solution eluting from a separation column, then atomizes and sprays the solution in an aerosol stream as droplets, which dry to form residue particles. A corona discharge source or needle selectively charges the residue particles as the aerosol stream enters a chamber. The selectively charged residue particles, each carrying a charge in proportion to its size, are collected at a conductive filter. The electrical current along a conductor coupled to the filter is measured to provide an indication of concentrations of the target compound.
The detection process as outlined above results in a waste mixture of gas and liquid exiting the nebulizer. The waste mixture is also well above atmospheric pressure because the system must operate under elevated pressure to atomize and spray the particles. The waste mixture is typically siphoned into sealed bottles ostensibly capable of withstanding pressure that may reach or exceed 3.5-psid.
The pressurized bottles can present myriad problems for running the HPLC system. The waste mixture typically flows from the nebulizer in uneven, alternating plugs of gas and liquid, and can cause pressure spikes that that create noise in the electronic detection signal of the nebulizer and attendant difficulty in accurately assessing the target compound concentration. In addition to the signal noise, the pressurized system can present safety issues. In many applications, the nebulizer waste contains hazardous solvents or particles. While the bottles are ostensibly designed to withstand pressure, the risk of explosion or leak is sufficiently high enough that many facilities cannot use the detection system. The bottle also takes up space which is often at a premium in the laboratory setting, and typically requires special handling disposal techniques that are more costly than simply routing the waste stream to a drain or to a remote container.
It is known that fluid flow through small diameter tubes can create a loss in pressure, as is described in U.S. Pat. No. 6,813,929 assigned to Dionex Corporation. Another potential solution may be to include pumps downstream of the nebulizer similar to those described in US Publication No. 2008/0154543 to Rajagopal. However, the small diameter tubes alone have proven insufficient for completely de-pressurizing a waste system or satisfactorily mitigating noise. Pumps may be effective for reducing pressure but introduce additional costs in equipment and maintenance and may not mitigate the problematic pressure spikes.
Other steps may be taken to reduce the inherent safety risk. For example, the waste bottle may be kept in a cage or similar sealed container. The addition of a cage/container, however, increases the footprint of the system and still may not meet safety codes. Simply minimizing safety risks also does nothing to eliminate noise in the detection signal.
A device that can simply and economically reduce the signal noise caused by pressure spikes and eliminate the need for pressurized waste containers would be welcome.